It is well known that degradation of solid organic waste material to a bioactive, stabilised end product such as compost for gardens, can be achieved by treating the solid organic waste material under either anaerobic or aerobic conditions in which anaerobic or aerobic microorganisms, respectively, metabolise the waste material to the end product.
Aerobic decomposition of solid organic waste material takes place in the presence of oxygen. Energy produced during aerobic decomposition is released as heat, the temperature of the material frequently rising to 75° C. under ambient conditions. The resulting solid end product is generally rich in nitrates which is a readily bioavailable source of nitrogen for plants. Thus the bio-available resultant end product is an excellent fertilising material for gardens and has commercial value as such.
Anaerobic digestion of solid organic waste takes place in the absence of oxygen. Typically, the solid organic waste must be heated to a mesophilic or thermophilic temperature range in order for anaerobic microbial metabolism to be optimised. Energy produced during anaerobic digestion is conserved as biogas, predominantly methane and carbon dioxide. The resultant solid end product is generally rich in ammonium salts.
Ammonium salts are not readily bio-available for uptake by plants. It is known, therefore, to treat residues, resulting from anaerobic digestion, with conditions under which aerobic decomposition will proceed. Thus, the material is converted to one rich in nitrates and which is of commercial value.
Systems have generally been designed to cater discretely for each type of degradation, although some systems have been designed to combine both anaerobic and aerobic decomposition processes.
German Patent Number 4440750 relates to an apparatus for raw material and energy recovery from biomass which has an anaerobic fermentation unit, an aerobic composting unit, a gasification unit and a power generating plant. The apparatus utilises byproducts from the anaerobic fermentation unit and the aerobic composting unit to work synergistically to provide reduced amounts of residues and to improve raw material and energy production.
International Patent Application Number WO 94/24071 discloses treatment of organic bioresidues especially from municipal and industrial wastes, including raw and/or cooked food residues, agricultural wastes and/or plant vegetable components. The bioresidues are first homogenised, fermented in an anaerobic reactor wherein the resulting biogas is removed, and then the residual solids are transferred to a composting chamber.
These and other similar systems provide discrete and separate chambers or vessels for aerobic decomposition and anaerobic digestion, respectively. Material which has undergone one set of conditions is transferred to a separate location to undergo a secondary processing phase. The transfer of material from one location to another is not efficient in terms of time, costs and labour.
The present invention seeks to overcome, at least in part, some of the aforementioned disadvantages.